Pigments that are used in polar environments must be readily dispersible, and must exhibit highly polar surface characteristics. The degree of polarity may extend to the degree of being hydrophilic. Accordingly, it is important for certain applications to be able to render hydrophilic or to increase the hydrophilicity of the surface of pigment particles.
It is desired to make pigments that have a small particle size distribution, are highly dispersible in many kinds of solvents including water, require little or no grinding energy and exhibit good chemical resistance. Preferably such pigments should be made by an environmentally safe procedure.
It is well known that polysiloxane materials have many useful attributes, such as surface tension energy reduction, high heat resistance, high chemical resistance, surface active properties, and dispersing ability, while being essentially nontoxic. Polysiloxanes are also useful because they exhibit good weatherability when used as coating materials and adhesives.
A U.S. Pat. No. 5,364,633 disclosed the use of modified poly(dimethylsiloxane)s, which have hydrophilic or ionic moieties on their chains, as surfactants to entrap water-soluble and water-insoluble substances. The patent uses long poly(alkyleneoxide) chains with hydroxyl moieties at the end as the hydrophilic functions. Such compounds are quite costly to design and produce.
Two Japanese published patent applications, JP4036370 and JP2218723, disclose coating inorganic pigments by using modified poly(dimethylsiloxane)s as chemically bound dispersing agents. They have functional group(s), such as amino, hydroxyl, alkoxyl or isocyanate, in one molecular terminal, and react with surface hydroxyl groups of the pigments to form a covalent bond. The surface coated pigments disperse well in non-aqueous solvent, especially silicon oil. They require severe reaction conditions and high production cost.
There are many ways to produce siloxane coated dry pigments. However many such approaches require high power grinding to minimize the particle size of the pigment because of adhesion or cross-linking between the particles.
Accordingly, a need exists for pigments that have one or more of the following properties: (1) exhibit a small particle size distribution, (2) have a high dispersibility in high polar solvents, including water, alcohols, ketones, ethers, and the like, (3) have a high dispersibility in low polar solvents, including oils and varnishes, (4) require little or no grinding energy, (5) have high chemical resistance and high color strength, and (6) can be manufactured in an environmentally safe manner.